The invention relates to a process for mixing a synthetic material with complemental raw materials such as a pigment, another synthetic material and an additive. The materials are introduced into a mixing vessel through inlet ports. Each material is introduced at a rate such that a specific mixing ratio is established. The materials are mixed with one another in the specific mixing ratio and the resultant mixture is delivered into a processing machine by way of an outlet port in the mixing vessel.
Synthetic materials (plastics) are usually mixed or blended with other complemental raw materials. The exclusive use of synthetic materials without an additive is very rare. In general, more than one complementary material is mixed with the synthetic material and the proportions of these complemental materials are very small. From the economic standpoint, therefore, it is extremely advantageous to blend the raw materials with one another in the form of a mixture. Since the mixing operation requires numerous manipulations and the work area is frequently contaminated in the process, more recently there has been a considerable demand for automatic metering, mixing, and blending of the raw materials. Because of the great multiplicity of the materials that are to be added, and because of the difficulty in transporting these materials in a clean condition, it is also most desirable that the automatic mixing devices be mounted individually on the corresponding processing machines.
It is a known practice to provide a mixing vessel directly in front of the raw-material-admission-port of a processing machine with a metering apparatus being made available for each material that is to be blended. Through this metering apparatus the material is proportioned properly and is introduced into the mixing vessel in such a way that a given mixing ratio will be established between the materials. The materials are mixed with one another in the mixing vessel and are then discharged therefrom. However, it has proven very difficult to carry out the metering, mixing, and blending in processing machines of small capacity, and in many cases it is absolutely impossible to achieve satisfactory results. In one conventional automatic device of the type mentioned, a batch mixer is used. This conventional procedural method involves problems in the accuracy of the composition ratio, in the latter's variability, and in equipment costs. These drawbacks were successfully overcome by a device with a vertical continuously operating mixing vessel which is described in Japanese Patent Application No. 81,760/72. But this type of mixing does not differ from the conventional method to this extent that a separate metering device must still be provided for each of the raw materials that are to be blended. Consequently, this known apparatus is too expensive for processing machines of small capacity.
In another Japanese Patent Application, No. 15,273/72, a process was proposed in which a liquid coloring agent that is made by dispersing a pigment in liquid or paste form is injected into a synthetic material which is supplied from a hopper for the material. This liquid coloring agent is filled into the inlet port of the processing machine by means of a metering pump at a rate that is synchronous with the velocity of the mass of the material that is treated in the processing machine. This process works only if the complemental material that is to be added is in liquid form. It was developed as an automatic dyeing method in which the pigment is introduced in liquefied condition. Consequently, it is necessary to provide only a metering apparatus for the liquid coloring agent. It is not necessary to provide a metering apparatus for the main raw material. In this way equipment costs can be reduced. This mixing apparatus has entered the field of technology as an automatic dyeing apparatus that is to be mounted on an individual processing machine of small capacity. Although this automatic dyeing process is economical and effective when only one coloring agent is to be introduced into the main raw material, problems are encountered because aside from the coloring agent no complemental materials can be mixed or added at the same time. Moreover, the coloring agents that are usable for this process are limited exclusively to liquid coloring agents so that frequently used pulverized pigments cannot be employed. This is another drawback to this known process.
In a conventional, automatically operating mixing apparatus in which all of the raw materials are introduced into a mixing vessel and are mixed with one another, various types of raw materials can be mixed or blended with one another at the same time regardless of whether they are present in a liquid, pulverulent, or granulated form. But for each raw material that is to be mixed, a separate combination is required which consists of a metering apparatus, a drive mechanism, and a governing device, so that the equipment cost are very high and this apparatus is therefore suitable only for processing machines of medium to large capacity.
In the above mentioned automatic dyeing process using a liquid coloring agent, the main material is always introduced into a supply line which extends from a hopper or silo above the mixing vessel and in which the main material drops down by gravity. In this way the main material arrives at the inlet port of the processing machine with a pressure column (to be referred to hereinafter as "pressure of the raw material column") and the complemental raw material, such as the liquid coloring agent, is injected into the main raw material and is mixed therewith at a constant rate. In this case the velocity of the mass (flow rate) of the material treated in the processing machine, for example, the number of injection cycles in an injection molding machine, is used as the means for actuating the metering mechanism for the raw material. After the main raw material and the complemental raw material have been thusly combined, an agitator or stirring apparatus may be provided, and this involves no problems. In other words, in this process mixing and blending can be carried out. It follows from this that if quantitative feeding of non-liquid, complemental materials, such as a powdered pigment and a reclaimed (recycled) product were possible in the above mentioned automatic dyeing process for liquid coloring agents, the result would be excellent process which operates at low cost. However, since the quantitative feeding of powdered materials is impossible or difficult, no such process has been successfully realized up to the present time.
When a liquid pigment is introduced into the pressure column of the main raw material, based upon the mass flow rate, it should be introduced near the delivery point. It has been assumed the delivery point should be located in a narrow passage in the vicinity of the inlet port of the processing machine. Then, compared to the flow rate of the material treated in the processing machine, i.e., the injection cycle, the amount of pigment administered a number of times at this narrow point is substantially less than the amount of pigment introduced into the mixing vessels of the larger capacity in other conventional processes. The pressure feeding of pigment in a highly accurate, measured meter is possible only when the pigment is in a fluid state.
If a high grade metering pump is used, a small amount of fluid ranging from 0.1 to 1 g can then be introduced under pressure into the main raw material with a high degree of accuracy. Since no overly great amounts are present, the fluid can then be enclosed to an adequate extent in vacuums which form between the particles of the main raw material; this is the case even when the raw material has not been pushed aside especially or forced up.
On the other hand, solids cannot be fed under pressure by means of metering devices and feeding devices with the exception of a special device, such as a screw conveyor. When a screw conveyor is used, it is possible to feed in a very small amount of pulverized pigment under pressure against the pressure column of the main raw material, but the metering accuracy of a screw conveyor is generally not very great and consequently no adequate accuracy can be expected for an automatic mixing process from the use of such a device, and such a process will not work in actual practice. When using a complemental raw material other than a pigment, such as a reclaimed product or a synthetic that differs from the main material, a large amount must be added which at times amounts to 50 to 70% of the total material. The pressure feeding of such a huge amount of complemental raw material against the pressure head of the main raw material can be accomplished only by forcing it into the stream of the main raw material. Such an amount cannot be fed in even with the use of a screw conveyor.
No process has as yet been developed with which complemental solid raw materials can be combined with the pressure column of the main raw materials in a manner similar to that described above for the automatic dyeing process for liquid coloring agents. It is an advantage of this invention to create a mixing process by means of which at very small cost (a) complemental solid raw materials can be quantitatively combined with the pressure column of the main raw material using small as well as large mixing ratios, (b) ordinary mixing devices for solids can be used as the mixing device for the complemental raw material, the said devices allowing the solids to be delivered only into the open, (c) the accuracy of the addition of each complemental raw material can be raised to a level that is adequate for the mixing purpose, and (d) a continuously operating mixing vessel can be used.